The journey of high-energy photons from blazar jets to the Fermi telescope
Lorenzo Sironi (ITC-Harvard → Assistant Prof. at Columbia U.) Sixth International Fermi Symposium, November 12th 2015 with: D. Giannios, M. Petropoulou, A. Spitkovsky
Outline
•The Dawn (i.e., the origin of non-thermal emission in blazar jets):
magnetic reconnection as the accelerator of non-thermal particles
•The Sunset (i.e., the fate of TeV photons from distant blazars):
blazar-induced plasma instabilities in the intergalactic medium
M 87
No approximations, full plasma physics of ions and electrons
Tiny length-scales (c/ωp) and time-scales (ωp-1) need to be resolved:
è huge simulations, limited time coverage
The PIC methodMove particles under
Lorentz force
Deposit current from particle motion in the
cells onto the grid
Solve for EM fields on the grid
Interpolate EM fields on the grid to the particles in
the cells
EM fields on the grid
particles in the cells
• Relativistic 3D e.m. PIC code TRISTAN-MP (Buneman 93, Spitkovsky 05, LS+ 13,14)
!p =
r4⇡ne2
m
The dawn of blazar photons
Powerful emission and hard spectra
(Ghisellini+ 14)
FSRQs @ z>1
FSRQs @ z<1
Blazar phenomenology:
(1) blazars are efficient emitters (radiated power ~ 10% of jet power)
(Celotti+ 08)
electron energy
magnetic energy
(2) rough energy equipartition between emitting particles and magnetic field
p . 2dn
d�/ ��p
(3) extended power-law distributions of the emitting particles, with hard slope
1ES 0414+009
hard GeV-TeV spectra
Internal dissipation in blazar jets
Internal shocks in blazars:
• trans-relativistic (γ0~a few)
• magnetized (σ>0.01)
• toroidal field around the jet → field ⊥ to the shock normal
θ~90°
γ0
B0
� =B2
0
4⇡�0n0mpc2
Internal Dissipation: Shocks or Reconnection?
BL Lac
(Marscher et al. 08)
Shock
Density
γβx
εB
Shocks: no turbulence → no accelerationσ=0.1 θ=90° γ0=15 e--e+ shock
No “returning” particles → No self-generated turbulence No self-generated turbulence → No particle acceleration
No “returning” particles
Momentum space
Strongly magnetized (σ>10-3) quasi-perp γ0≫1 shocks are poor particle accelerators:
σ is large → particles slide along field lines
θ is large → particles cannot outrun the shock
unless v>c (“superluminal” shock) → Fermi acceleration is generally suppressed
B0
θ
The Fermi process
(LS+ 13, LS & Spitkovsky 09,11)
γ
γ dn
/dγ
Maxwellian
vA ⇠ c
Relativistic magnetic reconnection
v
Xv
reconnection electric field
reconnecting field
reconnecting field
� =B2
0
4⇡n0mpc2� 1Relativistic Reconnection
Can relativistic reconnection self-consistently produce non-thermal particles?
Dynamics and particle spectrum
σ=10 electron-positron
Hierarchical reconnection
Density
B0
box expanding at c
εB
y
y
x
x
(LS & Spitkovsky 14)
• Reconnection is a hierarchical process of island formation and merging (e.g., Uzdensky 10).
• The field energy is transferred to the particles at the X-points, in between the magnetic islands.
σ=10 electron-positron
Density
εB
(LS & Spitkovsky 14)
εB-ε E
Hierarchical reconnection
• The current sheet breaks into a series of secondary islands (e.g., Loureiro+ 07, Bhattacharjee+ 09, Uzdensky+ 10, Huang & Bhattacharjee 12, Takamoto 13).
• The field energy is transferred to the particles at the X-points, in between the magnetic islands.
• Localized regions exist at the X-points where E>B.
• Inflow into the X-line is non-relativistic, at vin ~ 0.1 c (Lyutikov & Uzdensky 03, Lyubarsky 05)
• Outflow from the X-points is ultra-relativistic, reaching the Alfven speed
Density
Inflows and outflows
vA = c
r�
1 + �
Inflow speed vin/c
Outflow speed
σ=10 electron-positron
The particle energy spectrum
•p=2
Time →
σ=10 electron-positron
MB
• At late times, the particle spectrum in the current sheet approaches a power law dn/dγ∝γ-p of slope p~2.
• The normalization increases, as more and more particles enter the current sheet.
• The mean particle energy in the current sheet reaches ~σ/4
→ rough energy equipartition
• The max energy grows as γmax∝t (compare to γmax∝t1/2 in shocks).
•γmax∝t
Time [ωp-1](LS & Spitkovsky 14)
•γmax
• In 3D, the in-plane tearing mode and the out-of-plane drift-kink mode coexist. • The drift-kink mode is the fastest to grow, but the physics at late times is governed by the tearing mode, as in 2D.
drift-kink
tearing
(LS & Spitkovsky 14)
3D: particle spectrum
•γmax •γmax∝
t
Time →
Time [ωp-1]
• At late times, the particle spectrum approaches a power-law tail of slope p~2, extending in time to higher and higher energies. The same as in 2D.
• The maximum energy grows
as γmax∝t. The inflow speed /
reconnection rate is vin/c~ 0.02
in 3D (vs vin/c ~ 0.1 in 2D).
(LS & Spitkovsky 14)
2D in-plane
2D out-plane
σ=10 electron-positron
Density
The highest energy particles
γ
x
y
(LS & Spitkovsky 14)
Two acceleration phases: (1) at the X-point; (2) in between merging islands
• In cold plasmas, the particles are tied to field lines and they go through X-points.
• The particles are accelerated by the reconnection electric field at the X-points, and then advected into the nearest magnetic island.
• The energy gain can vary, depending on where the particles interact with the sheet.
(LS & Spitkovsky 14)
(1) Acceleration at X-points
Implications for blazar emission
Density
Color: Emission Regions
(1) Relativistic reconnection is efficient
(Ghisellini+ 14)
FSRQs @ z>1
FSRQs @ z<1
Blazar phenomenology:
• blazars are efficient emitters (radiated power ~ 10% of jet power)
Relativistic reconnection:
✓ it transfers up to ~ 50% of flow energy (electron-positron plasmas) or up to ~ 25% (electron-proton) to the emitting particles
(Sironi+ 15)
� =B2
0
4⇡n0mpc2
•electron-positron
•electron-proton
Efficiency
Bg/B0
☉ ☉☉ ☉
☉ ☉
(2) Equipartition of particles and fields
(Celotti+ 08)
electron energy
magnetic energy
Blazar phenomenology:
• rough energy equipartition between emitting particles and magnetic field
Relativistic reconnection:
✓ in the magnetic islands, it naturally results in rough energy equipartition between particles and magnetic field
(Sironi+ 15)
•electron-positron
•electron-proton
Equipartition parameter
Bg/B0
☉ ☉☉ ☉
☉ ☉
(3) Extended non-thermal distributions
p . 2dn
d�/ ��p
Blazar phenomenology:
• extended power-law distributions of the emitting particles, with hard slope
hard GeV-TeV spectra
Relativistic reconnection:
✓ it produces extended non-thermal tails of accelerated particles, whose power-law slope is harder than p=2 for high magnetizations (σ>10)
(LS & Spitkovsky 14, confirmed by Guo+ 14,15, Werner+ 14)
•p=4•p=3
•p=1.5
•p=2
γ
1ES 0414+009
The sunset of TeV photons
TeV photons are absorbed in the IGM
TeV photons from blazars pair-produce in the IGM by interacting with ~ eV EBL photons. • mean free path is ~100 Mpc
The beam of electron-positron pairs has:Lorentz factor γ=106-107 and density ratio α=10-15-10-18 (wrt the IGM plasma)
~100 Mpc ~100 kpc
Blazar TeV e--e+ pairs
These pairs should IC scatter off the CMB, producing ~ GeV photons. • mean free path is ~ 100 kpc (IC cooling length)
multi-GeV
No excess GeV emission from blazarsEvery TeV blazar should have a GeV halo of reprocessed light. However, not seen!
(Neronov & Vovk 10)
expected cascade emission
IGM fields or plasma instabilities
2) The pair energy is deposited into the IGM as heat, via collective plasma instabilities (Broderick, Chang & Pfrommer 12, 13, 14)
1) IGM magnetic fields deflect the streaming pairs (Neronov & Vovk 10, Tavecchio+ 11, Finke+15)
Every TeV blazar should have a GeV halo of reprocessed light. However, not seen! Two possibilities:
(Tavecchio+ 11)
reprocessed GeV emission
(Finke+ 15)
Plasma instabilities in the IGMInterpenetrating beams of charged particles are unstable (beam-plasma instabilities)
Blazar-induced relativistic pairs IGM plasma
Two-stream (bump on tail) instability
energy from waves to particles: → damping
energy from particles to waves: → instability
microscopic scales!
Oblique instability
beam
(LS & Giannios 14)
Beam-plasma linear evolutionLinear analysis: the oblique instability grows 10-100 times faster than the IC cooling losses.
The non-linear evolution of the beam-plasma system requires PIC simulations.
(Broderick et al. 12)
If all the beam energy is deposited into the IGM via plasma instabilities:
• No reprocessed blazar GeV emission
• IGM field estimates are invalid
• IGM heating from blazars will have important cosmological implications
(Chang et al. 12)
10% in heat, 90% in GeV emissionBlazar-induced beams: Lorentz factor γ=106-107 and density ratio α=10-15-10-18
Numerically tractable: Lorentz factor γ=101-103 and density ratio α=10-1-10-3
COLD (i.e., monoenergetic) beams:
• Regardless of the beam γ or α, the beam longitudinal momentum dispersion at the
end of the evolution reaches ~ 0.2 γ, and the IGM heating fraction reaches ~10%.
→ 90% is still available to power the reprocessed GeV emission.
IGM
hea
ting
fract
ion
(LS & Giannios 14)
Blazars beams are not cold
(Miniati et al 13)
Blazar beams are born WARM:
• the pair production cross section peaks at ~ few mec2. • the TeV blazar spectrum and the EBL spectrum are broad.
distance
The heating fraction will be ≪10%:
• if the initial longitudinal beam dispersion is already > 0.2 γ, as expected for blazar beams. IG
M h
eatin
g fra
ctio
n
(LS & Giannios 14)So, nearly all of the energy stays in the beam!
• The (failed) dawn: internal shocks, if significantly magnetized
(σ>10-3) and quasi-perpendicular, are poor particle accelerators.
• The (likely) dawn: magnetic reconnection in magnetically-
dominated flows (σ≫1) is fast and efficient, can produce non-
thermal populations with a power-law slope p~1÷2, and results in
rough energy equipartition between particles and fields.
• The sunset: TeV photons will pair-produce in the IGM. The
resulting beam will deposit ≪10% of the its energy into the IGM.
Most of the beam energy will result in multi-GeV emission by IC
scattering off the CMB.
B0
Summary High-energy emission from blazars:
θ~90°
Density
The highest energy particles
γ
x
y
(LS & Spitkovsky 14)
Two acceleration phases: (1) at the X-point; (2) in between merging islands